1. Field of Invention
This invention relates to a capacitive displacement transducer used for converting a displacement caused by a change of a physical quantity, such as pressure, into an electrical signal using electrostatic capacitances; and, more particularly, to a novel capacitive displacement transducer which is capable of eliminating adverse influence due to distributed capacitance, such as appearing between an electrode and a casing, to fixed capacitances, such as appearing between electrodes, and the like, and furthermore, which is capable of compensating for zero point fluctuations and span fluctuations, such as caused by changes in ambient temperature and static pressure.
2. Description of the Prior Art
Capacitive displacement transducers of the above type have been used, for example, to detect flow rate, pressures, etc, wherein the quantity to be measured causes changes of capacitance which are then converted into electrical signals which are in turn transmitted to remote receiving stations.
However, prior capacitive displacement transducers are deficient in that since there exists distributed capacitances between fixed and movable electrodes and a body forming the casing and/or between the fixed and movable electrodes, the conversion characteristics exhibit non-linearity and/or error of measurement arising from such distributed capacitances.
One solution to such deficiency is proposed in Japan Patent Application Ser. No. 57-26711 entitled "Capacitive Displacement Transducer" and corresponding U.S. Pat. No. 4,387,601 and is described herein in FIGS. 1, 2, 3 and 4.
FIG. 1 depicts a sensor section of a capacitive displacement transducer comprising a body 10 in the form of a single chamber configuration having two diaphragms 11, 12 disposed on either end surface of body 10 for receiving pressures P.sub.H and P.sub.L (see arrows) whose peripheries are welded to body 10. A hollow chamber surrounded by a through hole 13 bored in body 10 and by diaphragms 11,12 is filled with a sealing liquid 14, such as, for example, silicone oil.
In the central portion of the hollow chamber there are arranged a movable electrode 16 and fixed electrodes 17,18 disposed opposite to movable electrode 16 to form electrostatic capacitances C.sub.1 and C.sub.2. Each of these electrodes 16,17, 18, is supported at one end thereof by an insulating material disposed in body 10. A rod 19 passing through the hollow chamber couples together diaphragm 11, 12 at their central portions. The central portion of rod 19 is secured to movable electrode 16 in the inside of an electrode chamber. Displacement of diaphragms 11,12, in response to a differential pressure (P.sub.H -P.sub.L), is transferred to movable electrode 16 to thereby differentially displace capacitances C.sub.1 and C.sub.2.
FIG. 2 is an electrical equivalent circuit diagram of the sensor section of FIG. 1. Between movable electrode 16 and body 10 is formed a distributed capacitance C.sub.s. Between movable electrode 16 and fixed electrode 17,18 are formed fixed capacitances C.sub.F. These capacitances do not change regardless of the differential pressure (P.sub.H -P.sub.L). On the other hand, capacitances C.sub.1, C.sub.2 have capacitances which vary differentially in response to the differential pressure (P.sub.H -P.sub.L).
FIG. 3 depicts a displacement converting section for converting differential pressure into an electrical signal corresponding thereto, using the sensor section of FIG. 1, wherein the connection point, between capacitances C.sub.1 and C.sub.2, is connected to the input end (B) of an inverter G.sub.1. Between the output end (C) and the input end (B) of inverter G.sub.1 is connected a constant value current limiting circuit CC.sub.1 in a negative feedback circuit. Output end (C) of inverter G.sub.1 is connected first to input end C.sub.L of an n-bit counter CT.sub.1, whose output end Q.sub.n is connected through a NAND gate to fixed electrode 17 to form a first electrode of capacitor C.sub.1, and second through an inverter G.sub.3 and a NAND gate G.sub.4 to fixed electrode 18 to form a second electrode of capacitor C.sub.2. The other input end of each of NAND gates G.sub.2 and G.sub.4 is connected to output end (C) of inverter G.sub.1.
A first positive feedback loop acting on inverter G.sub.1 is formed by NAND gate G.sub.2 and capacitance C.sub.1. A second positive feedback loop acting on inverter G.sub.1 is formed by NAND gate G.sub.4 and capacitance C.sub.2. These feedback loops are alternately switched by means of the output of counter CT.sub.1 through NAND gates G.sub.2,G.sub.4 to sustain oscillation. The output (Q.sub.n) of counter CT.sub.1 is smoothed by a filter circuit FC.sub.1.
The operation of the foregoing will now be described with reference to FIG. 4 and ignoring fixed capacitance C.sub.F to simplify description. As shown in FIG. 4, line (A), when the output (A) of NAND gage G.sub.2 is at a high level H (H will be referred to herein as a high level) and provides a voltage +V.sub.z, by its leading edge the composite capacitance C.sub.t, comprising capacitance C.sub.1, distributed capacitance C.sub.s and capacitance C.sub.2, is charged in series, and the potential of the input end of inverter G.sub.1 reaches up quickly to a certain voltage and rises substantially vertically as shown in FIG. 4, line (B). Consequently, a change e.sub.1 of the terminal voltage of distributed capacitance C.sub.s, evaluated while taking a threshold level V.sub.TH of inverter G.sub.1 as a reference, is represented by the following equation: ##EQU1##
At this moment, the output (C) of inverter G.sub.1 is at a low level L (L will be referred to herein as a low level). Thus, since the constant value current limiting circuit CC.sub.1 is connected between the input (B) and the output (C) ends of inverter G.sub.1, the charges stored in distributed capacitance C.sub.s and capacitance C.sub.2 start immediately to discharge through constant value current limiting circuit CC.sub.1 and the output impedance of inverter G.sub.1. However, since a discharge current i, caused by the foregoing discharge action, is restricted to a certain value by constant value current limiting circuit CC.sub.1, the voltage at the input end of inverter G.sub.1 is lowered linearly, as shown in FIG. 4, line (B). A discharge time t.sub.1 required for that voltage to go down to threshold level V.sub.TH is given by the following. EQU it.sub.1 =e.sub.1 (C.sub.1 +C.sub.t) (2)
By combining equations (1) and (2), the following is obtained. ##EQU2##
When that voltage goes down to threshold level V.sub.TH of inverter G.sub.1, the output (C) of inverter G.sub.1 is inverted and assumes the H level (i.e. high level). (See FIG. 4, line (C)). As a result, output (A) and NAND gate G.sub.2 assumes a low level L, and the voltage at the input end of inverter G.sub.1 takes a value e.sub.1 ' identical to that given by equation (1), but opposite in polarity. Thereafter, discharge action of opposite polarity takes place linearly by means of constant value current limiting circuit CC.sub.1. Subsequently, when threshold level V.sub.TH of inverter G.sub.1 is attained, output (C) of inverter G.sub.1 is inverted, as shown in FIG. 4, line (C). Since this discharge action of opposite polarity takes place also with current i of a certain value, discharge time t.sub.1 ' becomes identical to t.sub.1, namely, EQU t.sub.1 =t.sub.1 ' (4)
The foregoing relationships are maintained even after counter CT.sub.1 has counted a given number and the circuit has been switched to the side of capacitor C.sub.2 by means of the output of counter CT.sub.1. Thus, the following equation holds. ##EQU3##
Therefore, the H period of a pulse signal obtained from output Q.sub.n of counter CT.sub.1 corresponds to capacitor C.sub.1 and the L period to capacitor C.sub.2. By averaging these periods using filter circuit FC.sub.1 there is obtained a calculated result, C.sub.1 /(C.sub.1 +C.sub.2) related to the duty ratio of the pulse signal. This calcuated result gives a value proportional to the displacement of movable electrode 16, i.e. the differential pressure (P.sub.H -P.sub.L). In addition, distributed capacitance C.sub.s is eliminated.
For reference, the fixed capacitance C.sub.F existing across the capacitances C.sub.1 and C.sub.2 can be eliminated by connecting fixed condensers in parallel across circuit CC.sub.1 and making their capacitances identical to those of the fixed capacitance C.sub.F.
Further, to compensate for any error resulting from change of ambient temperature or static pressure on the sensor section, the conventional displacement transducer is equipped with an independent temperature sensor for measuring the temperature of body 10 and another independent pressure sensor for measuring the pressure, i.e. static pressure, of the sealing liquid 14. Specifically, the outputs of these sensors are applied to a compesation voltage generating circuit and converted thereby into a zero compensation temperature signal and a zero compensation span signal. These zero compensation signals are algebraically added to the output of an arithmetic circuit for adding and calculating the differential pressure (P.sub.H -P.sub.L), whereby a fluctuation of the zero point resulting from a fluctuation of temperature of static pressure is compensated.
In case a span fluctuation resulting from a fluctuation of temperature or static pressure is not negligible, a span compensation temperature signal and a span compensation pressure signal are generated by the foregoing compensation voltage generating circuit to change the voltage/current conversion gain of an output circuit and to thereby compensate for fluctuation of the span.
To perform such compensation, the temperature sensor and pressure sensor must be located inside body 10. A conventional system having a temperature sensor disposed inside a body is disclosed, for example, in Japan UM Laid Open No. 55-13317, and another conventional system having a pressure sensor disposed inside a body is disclosed, for example in Japan Laid Open No. 54-67480.
Although the transducer shown in FIGS. 1-3, is capable of eliminating adverse influences due to distributed capacitance C.sub.s and fixed capacitance C.sub.F, disadvantageously, signals relating to the H time duration and the L time duration of the output of counter CT.sub.1, which are proporational to the capacitances of capacitors C.sub.1 and C.sub.2, are determined depending upon the constant value current characteristics of limiting circuit CC.sub.1. Thus, if the performance of constant value current limiting circuit CC.sub.1 is degraded, errors are likely to be produced. For example, if circuit CC.sub.1 is degraded and a degradation resistance R.sub.cc indicated by the dotted line in FIG. 3 is equivalently formed in the circuit, voltage changes e.sub.1 and e.sub.2 at the input end (B) of inverter G.sub.1 arising when the circuit is switched over to the side of capacitor C.sub.1 and to the side of capacitor C.sub.2 become different from each other. Thus, current flowing through degradation resistor R.sub.cc, while bypassing constant value current limiting circuit CC.sub.1, differs between the side of capacitor C.sub.1 and the side of capacitor C.sub.2, and accordingly produces an error.
Furthermore, to eliminate fixed capacitance C.sub.F, fixed condensers are inserted across either end of constant value current limiting circuit CC.sub.1. However, practically, stray capacitances are also formed at either end of circuit CC.sub.1. These capacitances change their capacitances in response to temperature variations. Thus, it is impossible to eliminate completely the fixed capacitance C.sub.F and therefore errors are produced due to non-linearity.
Moreover, disadvantageously, the conventional transducer includes a temperature sensor and a pressure sensor disposed in the body in order to compensate for errors caused by changes in ambient temperature and static pressure. Thus, the conventional configuration of the sensor section is complicated and expensive.
Thus, in the prior art, there is a need for an inexpensive, reliable and simple displacement type transducer which can compensate for changes in ambient temperature and static pressure.